Zooplankton-phytoplankton biomass and diversity relationships in the Great Lakes.

Quantifying the relationship between phytoplankton and zooplankton may offer insight into zooplankton sensitivity to shifting phytoplankton assemblages and the potential impacts of producer-consumer decoupling on the rest of the food web. We analyzed 18 years (2001-2018) of paired phytoplankton and zooplankton samples collected as part of the United States Environmental Protection Agency (U.S. EPA) Great Lakes Biology Monitoring Program to examine both the long-term and seasonal relationships between zooplankton and phytoplankton across all five Laurentian Great Lakes. We also analyzed effects of phytoplankton diversity on zooplankton biomass, diversity, and predator-prey (zooplanktivore/grazer) ratios. Across the Great Lakes, there was a weak positive correlation between total algal biovolume and zooplankton biomass in both spring and summer. The relationship was weaker and not consistently positive within individual lakes. These trends were consistent over time, providing no evidence of increasing decoupling over the study period. Zooplankton biomass was weakly negatively correlated with algal diversity across lakes, whereas zooplankton diversity was unaffected. These relationships did not change when we considered only the edible phytoplankton fraction, possibly due to the high correlation between total and edible phytoplankton biovolume in most of these lakes. Lack of strong coupling between these producer and consumer assemblages may be related to lagging responses by the consumers, top-down effects from higher-level consumers, or other confounding factors. These results underscore the difficulty in predicting higher trophic level responses, including zooplankton, from changes in phytoplankton assemblages.

Reevaluation of the genus Cyclops Mller, 1776 (Cyclopoida: Cyclopidae) in the Laurentian Great Lakes basin: first report of the Palearctic species Cyclops divergens Lindberg, 1936 from Lake Erie and documentation of Cyclops sibiricus Lindberg, 1949 in the St. Marys River.

Large cyclopoid copepods of the genus Cyclops Mller, 1776 are seldom collected in the Laurentian Great Lakes, with only Cyclops scutifer Sars, 1863 and Cyclops strenuus Fischer, 1851 reported from the region. Rare reports of the species C. strenuus date back to 1972 within the Great Lakes basin. The first specimens reported as C. strenuus were collected from the St. Marys River, and additional specimens have been collected from western Lake Erie since 2013. We examined all available archived materials of C. strenuus from the Great Lakes and determined that specimens from the two localities belong to two separate species, neither of which refer to C. strenuus. Archived specimens collected from the St. Marys River in 1972 and 1995 were reidentified as Cyclops sibiricus Lindberg, 1949, a Holarctic species known from Siberia, Russian Federation, Alaska, USA, and northern regions of Canada. The occurrences of C. sibiricus from the St. Marys River extend the known distribution of the species southward some 1,688 km in the Nearctic region. Cyclops specimens collected from the western basin of Lake Erie in 2013, 2014, and 2019 were identified as the Palearctic species Cyclops divergens Lindberg, 1936 using both conventional taxonomy and genetic barcoding. C. divergens is known from localities across much of Europe and eastward into Central Asia. The occurrences of the species from western Lake Erie constitute the first detection of C. divergens in the Great Lakes and the Nearctic region. Therefore, we expect C. strenuus does not occur in the Great Lakes basin and is likely restricted to the Palearctic region.

Deep chlorophyll maxima across a trophic state gradient: A case study in the Laurentian Great Lakes.

Deep chlorophyll maxima (DCM) are common in stratified lakes and oceans, and phytoplankton growth within DCM often contributes significantly to total system production. Theory suggests that properties of DCM should be predictable by trophic state, with DCM becoming deeper, broader, and less productive with greater oligotrophy. However, rigorous tests of these expectations are lacking in freshwater systems. We use data generated by the U.S. EPA from 1996 to 2017, including in situ profile data for temperature, photosynthetically active radiation (PAR), chlorophyll, beam attenuation (c p), and dissolved oxygen (DO), to investigate patterns in DCM across lakes and over time. We consider trophic state, 1% PAR depth (z 1%), thermal structure, and degree of photoacclimation as potential drivers of DCM characteristics. DCM depth and thickness generally increased while DCM chlorophyll concentration decreased with decreasing trophic state index (greater oligotrophy). The z 1% was a stronger predictor of DCM depth than thermal structure. DCM in meso-oligotrophic waters were closely aligned with maxima in c p and DO saturation, suggesting they are autotrophically productive. However, the depths of these maxima diverged in ultra-oligotrophic waters, with DCM occurring deepest. This is likely a consequence of photoacclimation in high-transparency waters, where c p can be a better proxy for phytoplankton biomass than chlorophyll. Our results are generally consistent with expectations from DCM theory, but they also identify specific gaps in our understanding of DCM in lakes, including the causes of multiple DCM, the importance of nutriclines, and the processes forming DCM at higher light levels than expected.